An important risk factor for relapse to smoking is female gender. Women have less success in unaided quit attempts and may be less responsive to nicotine replacement therapies than men. Higher stress reactivity and prevalence of mood-related symptoms are increasingly recognized as important for the increased difficulty in quitting experienced by women. Both norepinephrine and acetylcholine are critical neurotransmitters regulating activity in brain circuits essential for stress-related behaviors, including the amygdala-prefrontal cortical circuit. In keeping with the overarching theme ofthe Yale-SCOR related to sex differences in noradrenergic control of smoking behavior, we hypothesize that the disparity in susceptibility to smoking relapse between women and men may be due to underlying differences in cholinergic- noradrenergic interactions. Thus, it is important to examine these mechanisms in an animal model with input and feedback from Projects II and III to determine what alterations in neurochemistry might underlie these differences and to provide an integrated understanding ofthe neurobiological basis for sex differences in smoking. The primary aims of Project I are to identify the brain areas regulated by guanfacine and nicotinic drugs under conditions related to stress-reactivity (Aim 1), to determine whether stimulation ofthe noradrenergic system with guanfacine can decrease stress reactivity (Aim 2) and dopamine dependent behaviors (Aim 3) in a hypercholinergic model of increased stress reactivity in male and female mice. This project will also determine whether the effects of guanfacine on anxiety and nicotine reinforcement depend on expression of nAChRs (Aim 4). This basic science project is designed to identify neuronal mechanisms underlying similar effects of guanfacine to decrease smoking in female and male human subjects, but to decrease stress reactivity only in women. It is also designed to identify neurochemical mechanisms underlying the effects of guanfacine on the cholinergic and dopaminergic systems. The findings from Project I will be integrated with those from Projects II and III of the Yale-SCOR to inform the development of novel gender-sensitive therapeutics for smoking cessation RELEVANCE (See instructions); Tobacco smoking is the most significant preventable cause of death. Despite availability of therapies, women are more likely to relapse to smoking when they try to quit. Stress reactivity in women could be responsible for this difference. The noradrenergic medication guanfacine helps women resist stress-induced relapse and helps women and men quit smoking. Understanding the neurobiology underlying these effects could pave the way for more tailored therapies for smoking cessation in women and men.